Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

Abstract

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

Keywords: SERS; enhanced fluorescence; enhanced spectroscopy; localized surface plasmons; nanoporous; nanoporous metals; photocatalysis; plasmonics.

Scheme 1. Diversity of Nanoporous Metals (NPMs) Including Various Material Selection (Au, Ag, Cu, Al, Mg, Pt, Rh), Design and Modeling (Finite Element Method (FEM), Finite-Difference Time-Domain (FDTD) Method, Effective Medium Approximation (EMA)), Plasmonic Properties (Near-Field Properties and SPR Spectra Tuning), Preparation Strategies (Dealoying, Templating, Galvanic Replacement Reaction, and Physical Vapor Deposition), Microcharacterization (Scanning Electron Microscopy (SEM), Tunneling Electron Microscopy (TEM), Scanning Transmission Electron Microscopy (STEM)), and Functionalization of the NPMs (SERS, Metal-Enhanced Fluorescence, and Photocatalysis)

Adapted with permission from ref (25), copyright 2011 American Chemical Society; ref (26), copyright 2018 Wiley-VCH; ref (27), copyright 2014 AIP Publishing LLC; ref (28), copyright 2020 Elsevier B.V; ref (29), copyright 2015 American Chemical Society; and ref (30), copyright 2016 American Chemical Society.

Figure 1

Preparation of nanoporous metals (NPMs). (A) Fabrication steps to prepare nanoporous gold (NPG) disks using the combination of the lithographic patterning and atomic dealloying. (a) Formation of a monolayer of polystyrene (PS) beads on an alloy-coated substrate; (b) O₂ plasma shrinkage of the PS beads; (c) Ar sputter-etching to form isolated alloy disks; (d) removal of PS beads; and (e) formation of NPG disks by dealloying. (f–j) SEM images taken at each process step. Adapted from ref (34). Copyright 2019 American Chemical Society. (B) Preparation of disordered NPAg thin films by thermally assisted dewetting method. (a) Schematic representation of the NPAg film defined by a pore width, W_NP. (b) Schematic of Ag film dewetting mechanism that resulted in NPAg formation during the thermal annealing. According to the Young equation (γ_s = γ_i + γ_f·cos θ), the metal remains as a continuous flat film only when surface energy of the bare substrate (γ_s) is larger or equal to the sum of the surface energy of the metal film (γ_f) and the substrate–metal interface energy (γ_i) at some contact angle θ between the silver surface and silver–substrate interface. (c) Porosities and pore widths of different NPAg sample types. Adapted with permission from ref (36). Copyright 2015 Wiley-VCH. (C) Preparation of mesoporous Rh nanostructures via chemical reduction on self-assemble polymeric poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA) micelle templates. Adapted with permission under a Creative Commons Attribution 4.0 International License from ref (55). Copyright 2017 The Authors. (D) Fabrication of nanoporous metallic networks based on the physical vapor deposition (PVD) strategy. (a) Illustration of the fabrication process of nanoporous metallic networks. Vapored metallic atoms are directly self-organized into a 3D network of nanoscale features on top of the nanoporous silica aerogel substrate. The metallic vapor is produced by PVD, either by sputtering or by evaporation. 3D SEM images of (b) gold and (c) silver networks. Adapted with permission from ref (59). Copyright 2016 Wiley-VCH.

Figure 2

Modeling the optical responses of nanoporous metal particles, networks, and films. (A) Finite-difference time-domain (FDTD) method-based modeling of nanoporous metal particles. (a) Schematic illustration of nanoporous gold (NPG) disk. (b) FDTD simulated electric field distribution of NPG disk with 500 nm diameter and 75 nm thickness. Adapted with permission from ref (33). Copyright 2014 The Royal Society of Chemistry. (c) FDTD calculated extinction spectra of nanoporous gold nanoparticles (NPG NPs) with various particle sizes, 66% particle volume porosity, and 20 nm pore size. Adapted from ref (72). Copyright 2017 American Chemical Society. (B) Effective medium approximation of nanoporous metal films by a cubic grid of gold wire model. (a) Model system consisting of a cubic grid of gold wires with two submodels; the first submodel considers only wires oriented parallel to the electric field, whereas the second submodel considers the orthogonal wires. When linearly polarized wave illuminates a thin film of the model material under normal incidence, the wires oriented parallel to E-field vector contribute differently to the spectrum as compared to the orthogonal ones. (b) Calculated transmission through the effective medium of the cubic gold network for various permittivities of the embedding medium. The inset shows SEM of typical nanoporous gold film. Adapted with permission from ref (27). Copyright 2014 AIP Publishing LLC. (C) Drude–Lorentz modeling of nanoporous gold film. (a) Fractal analysis of the SEM image of gold film. The fractal dimension (D_f), which can be directly computed from SEM images using the box-counting method that assigns a 0 or 1 value to each pixel in the SEM image, is the key morphological parameter to predict the plasmonic properties of NPG and simply tune them at will with the dealloying time. (b) Connection between the effective Drude model and the fractal analysis. Adapted from ref (11). Copyright 2018 American Chemical Society.

Figure 3

Tunable plasmonic properties of nanoporous gold (NPG) and nanoporous silver (NPAg) structures. (A) Tuning localized surface plasmon resonance (LSPR) of thin NPG film. (a) Illustration of the average sizes of ligaments and pores in typical NPG film. Adapted with permission from ref (8). Copyright 2014 AIP Publishing LLC. (b) Extinction spectra and (c) corresponding two resonant peak positions (λ₁ and λ₂) of NPG film with the pore sizes of 10–50 nm in water. The dashed red line in (c) represents the size-dependent resonance band of gold nanoparticles. Reproduced with permission from ref (7). Copyright 2011 AIP Publishing LLC. (B) Plasmonic properties of NPG disks with tunable plasmon resonances. (a) SEM image of high-density NPG disks with diameters of 400 nm fabricated on Si substrate (with the scale bar of 500 nm). (b) Extinction spectra of 400 nm diameter and 75 nm thick Au disks and NPG disks on glass substrates measured in air. The inset shows the in-plane and out-of-plane resonance modes. (c) Size-dependent extinction spectra of NPG disks with different diameters (300, 400, 500, and 700 nm) consisted of high-density NPG disk monolayers on glass substrates in air. Reproduced with permission from ref (33). Copyright 2014 The Royal Society of Chemistry. (C) Optical properties of NPAg films. (a) Representative SEM image of the NPAg formed by annealing a 50 nm thick Ag film on chromium-coated glass substrate. (b) Scattering and (c) transmittance spectra of different NPAg films. Adapted with permission from ref (36). Copyright 2015 Wiley-VCH.

Figure 4

Optical and plasmonic properties of nanoporous copper (NPC) and nanoporous aluminum (NPA). (A) Structural characterization and electric field distributions of hierarchical nanoporous copper (HNPC) structures. (a) Scanning transmission electron microscopy (STEM) image of Mg₇₂Cu₂₈ alloy ribbon, where the selected area electron diffraction of which shown in the inset demonstrates that (111), (242), and (333) correspond to CuMg₂ phase whereas (101) to Mg phase. (b,c) Element mappings elucidate the dark phase in (a) is Mg while the bright phase is CuMg₂. (d) Schematic of the HNPC used to simulate electric field distributions. (e) Typical electric field distribution (|E|/|E₀|) of the HNPC on the top surface of the ligament with pore size of 10 nm. Reproduced with permission from ref (21). Copyright 2018 Wiley-VCH. (B) Optical and plasmonic properties of pure aluminum, aluminum oxides, aluminum alloys, and nanoporous aluminum structures. (a) Plasmon resonance tuning ranges of the most common plasmonic materials, Au and Ag, compared with Al. (b) Calculated spectra for Al nanodisk (35 nm thick and 50 nm diameter) of (i) a pure Al, isolated Al nanodisk (black line); (ii) an isolated Al nanodisk with a 3 nm surface oxide (green); and (iii) the same Al nanodisk on an infinite SiO₂ substrate (orange). Reproduced from ref (22). Copyright 2014 American Chemical Society. (c) Reflectance spectra of pure Al, aluminum oxide, and nanoporous Al films. (d) Numerically computed local field enhancement of NPA film calculated at an excitation wavelength of 260 nm. Reproduced with permission under a Creative Commons Attribution (CC BY) License from ref (10). Copyright 2020 MDPI. (e) Imported map of the horizontal cross section of the as-prepared nanoporous aluminum–magnesium alloy (NPAM). (f) Electromagnetic calculations of field confinement (a.u.) of the NPAM film calculated at an excitation wavelength of 260 nm. Reproduced from ref (9). Copyright 2019 American Chemical Society.

Figure 5

Nanoporous metal substrates for surface-enhanced Raman spectroscopy (SERS). (A) Wrinkled nanoporous gold film SERS substrates. (a) Schematic diagram of the preparation of the wrinkled nanoporous gold film by thermal contraction of polymer substrate. (b) Microstructure of as-prepared NPG and (c) zoom-in SEM micrograph of wrinkled NPG films with nanopore sizes of 12 nm. (d) Comparison of CV SERS spectra based on the wrinkled NPG (w-NPG) and as-prepared NPGs with different pore sizes. (e) SERS spectra from the w-NPG with different pore sizes of 12 nm (w-NPG12), 26 nm (w-NPG26), and 38 nm (w-NPG38). The excitation wavelength is 632.8 nm. Reproduced from ref (25). Copyright 2011 American Chemical Society. (B) Aluminum for deep ultraviolet surface-enhanced Raman spectroscopy (DUV-SERS). (a) Schematic representation of nanovoid aluminum films. (b) SEM image of top view and 45° angled-view (inset) of the Al nanovoids. (c) (i) UV-SERS spectrum of a 1 mM adenine solution on a 200 nm void structured aluminum surface (red) compared to the UV-SERS spectrum on an evaporated aluminum surface (blue) and the resonant Raman spectrum of adenine solution without a plasmonic surface (black). The inset shows the structure formula of adenine. (ii) UV resonant Raman spectrum of bulk adenine in powder form. (iii) NIR SERS spectrum (excitation 785 nm) of adenine solution on Klarite. Reproduced from ref (153). Copyright 2013 American Chemical Society. (d) UV Raman spectra of salmon sperm DNA deposited on rough Al substrate (red curve) and nanoporous Al (NPA) substrate (blue curve). The inset show SEM of typical NPA film. Reproduced with permission under a Creative Commons Attribution (CC BY) License from ref (10). Copyright 2020 MDPI.

Figure 6

Nanoporous gold (NPG) platforms for high sensitivity IR plasmonic sensing. (A) NPG metamaterials sensing performance with different surface coating. (a) Schematic of sensing approach: a near-infrared light beam illuminates the nanoporous gold, and the reflectance is measured around the plasma edge region where a significant spectral shift can be detected as a function of the number of molecules deposited on top of the material surface. (b) Reflectance curve of the NPG film as a function of the thickness of SiO₂ layer. (c) Corresponding spectral shift measured at R(0.85%). Reproduced with permission from ref (89). Copyright 2019 The Royal Society of Chemistry. (B) 3D NPG antenna for IR sensing. (a) SEM micrographs of the NPG vertical antenna. The inset displays title view of the same structure prepared in homogeneous gold (Au antenna). (b) Reflectance curves of 3D antenna arrays for NPG (black curves) and homogeneous gold (blue curves). (c) Corresponding sensitivity of the NPG and homogeneous Au antenna arrays. Reproduced with permission from ref (161). Copyright 2019 OSA Publishing.

Figure 7

Nanoporous metal-based fluorescence emission enhancement, imaging, and biosensing. (A) Distance-dependent plasmon-enhanced fluorescence of single fluorescent molecules. Reproduced from ref (182). Copyright 2015 American Chemical Society. (B) Overlaid image (40 × 40 μm²) of fluorescence emission over a surface contour on a NPG film. The shaded contour represents surface feature derived from the reflectance image recorded from the same region. Reproduced with permission from ref (172). Copyright 2013 The Royal Society of Chemistry. (C) Nanoporous gold leaf (NPGL)-based assay for virus detection. (a) Schematic of the pandemic influenza virus A (H1N1) detection using hybrid structure of quantum dots (QDs) and nanoporous gold leaf. The NPGL (i) and QDs (ii) were initially conjugated with antihemagglutinin (HA) antibodies (anti-HA Ab, Y-shape) by the reaction of ethylcarbodiimide (EDC)/N-hydroxysuccinimide (NHS). Then anti-HA Ab-conjugated with NPGL and QDs forms a complex (iii) in the presence of HA on the surface of influenza virus, finally enhancing PL intensity. (b) Fluorescence microscopic image of QDs on metallic nanostripe patterns. (c) PL enhancement corresponding to different quantities of recombinant influenza HA (H1N1) on anti-HA Ab-conjugated NPGL05. Inset: Calibration curve of PL intensity versus HA concentration. Reproduced with permission from ref (173). Copyright 2014 Elsevier B.V.

Figure 8

Extraordinary light transmission (EOT) effect and Fano resonances in nanoporous gold nanohole arrays. Schematic of SPP and LSP weak coupling (a) and strong coupling (b), leading to transmission dip and peak is shown. (c) Coupled oscillator model, a mechanical analogue of the three coupled plasmonic resonances, is shown. (d) Fano-resonant EOT spectra with highly dispersive character emerges from the oscillator model. (e) SEM image of nanoporous plasmonic nanohole arrays are shown. Nanoporous features are significantly smaller with respect to nanohole openings. (f) Fano-resonant EOT transmission spectra obtained from broadband transmission measurements are shown. Adapted with permission from ref (207). Copyright 2020 Elsevier B.V.

Figure 9

Plasmon-assisted photocatalysis based on nanoporous gold (NPG). (A) Schematics of plasmon-induced hot electron/hole pairs in a NPG catalyst and the mechanistic representation of hole-assisted electro-oxidation of methanol on Au ligaments by pumping away hot electrons from the Schottky barrier-free plasmonic catalyst. (B) Direct plasmon-enhanced electro-oxidation of methanol catalyzed by a surface-engineered NPG catalyst. The graph depicts cyclic voltammetry curves of a methanol oxidation reaction on {111}-rich NPG and 34.1 nm NPG in a 0.5 M KOH/1.0 M methanol solution with and without light illumination (scan rate: 10 mV s^–1). The inset shows typical SEM image of NPG film. Adapted with permission from ref (228). Copyright 2018 Elsevier Ltd. (C) Plasmonic heating-enhanced electrochemical current in NPG cathodes. (a) Schematic of the photoelectrochemical current enhancement setup. The inset shows typical scanning electron micrograph of NPG synthesized on glass slide substrate. (b) Comparison of the photoelectrochemical current density generated by external heat only (red) and by both plasmonic and external heats (green). Reproduced with permission under a Creative Commons Attribution 4.0 License (CC BY) from ref (229). Copyright 2019 IOP Science.